29th International Cosmic Ray Conference Pune (2005) 00, 101–104

Characterization of the PMTs for the Telescope Array experiment
S. Yoshida for the TA collaboration(a) Dept. of Physics, Chiba University, Chiba 263-8522 Japan

Presenter: S. Yoshida (syoshida@hepburn.s.chiba-u.ac.jp), jap-yoshida-S-abs1-he15-poster
The Telescope Array (TA) experiment is being constructed in Utah, USA in order to confirm or refute theexistence of super-GZK cosmic rays observed by the AGASA shower array. This paper describes the methodsof calibration and characterization in the laboratory for TA PMTs used for the ground array detectors. Theirgain, charge response, and their dynamic range have been extensively scanned and calibrated. All the results
have been combined and provided in the form of the data library for being implemented by the detector Monte
Carlo simulation. The prospect of contributions of the present calibration to the TA event analysis is also brieflymentioned.

1.

Introduction

The Telescope Array (TA) is a high energy cosmic ray hybrid detector which consists of a ground array covering
approximately 900 km and three stations of florescence detectors. They are deployed in desert of western
Utah, USA. The surface detector (SD) in the array uses two layers of scintillation counters viewed by two
photomultiplier tubes (PMTs). A majority of TA events consists of particle density and timing profile recorded
in individual surface detectors. An accurate understanding of the PMT response is, therefore, necessary inorder to determine both the geometry of these events as well as to reconstruct their energies. In this paper we
describe our relative and absolute calibration of the PMTs for the SDs in the laboratory.

2.

PMT calibration system

Figure 1 illustrates the calibration system for the PMTs. Seven PMTs put in a dark box are illuminated bydiffuse light from UV LEDs for measuring the PMT gain, single photoelectron (SPE) response, and darkcount rate. Signals from the PMTs are read out by the CAMAC ADC module, the FADC module used in thefront-end electronics of the TA experiment, and the digital scope, depending on different aspects of the PMTcharacterization. For example the data taken by the scope indicated that a SPE waveform has been found to be
represented well by double-Gaussian and variations of the waveform from tube to tube can be negligible.

3.

SPE response

The charge response for SPE detection has been extensively studied. Figure 2 in the left panel shows an
example of the calibration data. Using a PMT amplifier with gain of leads to a clear SPE peak. The
peak-to-valley ratio (P/V) is 1.5. This SPE peak has been used as an indicator of the PMT gain, which allows tostudy the gain - HV scan. The right panel of Figure 2 shows an example. We scanned all the PMTs to confirm
they satisfy the following criteria:(1) The gain of   

can be realized by high voltage lower than 1600 volts.

(2) P/V is better than 1.
More than 95 % of the tubes we have scanned passd these requirements. The numbers like P/V, HV dependences of gain, and time width of SPE waveform are recorded into a database, which is used by the detector
Monte Carlo simulation.

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S. Yoshida et al.
RS−232CUtahFlasherLED

ICRR FADC

ND Filter Diffuser

LANPMTPMTADCCAMAC

LV Suppl.

Program.

Agilent

Pulser

Program.

Agilent

PulserMortor
Cntl?

Agilent
DigitalScope

500MHzGPIB

Gate Gen.

Logic Pulser

NIMLAN
GPIB
CAMAC
LOGIC

Figure 1. Left panel: Schematic view of the PMT calibration system.

Figure 2. Left panel:The charge histogram of a PMT illuminated by a dim UV LED light. One can see a SPE peakaccompanied by the pedestal peak. The PMT gain in this plot is   , which equals to the planned number in the actual
operation. Right panel:Gain of a PMT as a function of applied high voltage.Characterization of the PMTs for the Telescope Array PMT calibration

103Figure 3. PMT linearity data. The output charge is plotted as a function of the number of photoelectrons corresponding tothe LED luminosity. The PMT gain is   .

4.

Dynamic range

It is also important to measure the linearity of the PMT response to multiple photoelectrons. A bright LEDwith a set of calibrated neutral density filters is used to provide various luminosities of UV photons for sucha linearity measurement. The result is shown in Figure 3. The number of photoelectrons is estimated fromthe SPE peak and the attenuation factor of each filter. It is found that an TA PMT has a linear response up to 
approximately 500 photoelectrons at the gain of . Since a representative FWHM of the LED light pulse
is 30 nsec in this measurement, the corresponding current where the PMT deviates from the linear response is mA.